Texturing has been used as a technique for light trapping and to improve the efficiency of photodetectors and solar cells due to multiple internal reflections and light trapping. A portion of the known literature describes the use of backside texturing of photodiodes to improve the absorption of near infrared light energy. One of the first descriptions of using this technique in photodetectors was by A. E. St. John in U.S. Pat. No. 3,487,223, “Multiple Internal Reflection Structure in a Silicon Detector which is Obtained by Sandblasting”. Another description was provided by J. E. Cotter, “Optical intensity of light in layers of silicon with rear diffuse reflectors,” Journal of Applied Physics, vol. 84, no. 1, pp. 618-24, 1 Jul. 1998. A more recent description of the same technique on ultra-thin solar cells has been given by O. Berger, D. Inns and A. G. Aberle, “Commercial white paint as back surface reflector for thin-film solar cells,” Solar Energy Materials & Solar Cells, vol. 91, pp. 1215-1221, 2007, hereinafter referred to as Aberle. Aberle disclosed the use of white paint as the backside diffuse reflector. Aberle's initial results show a large increase in the absorption with the white paint as a back surface reflector but only a modest ten or twenty percent increase in the quantum efficiency. Aberle's later results indicate that a twenty to forty percent enhancement in conversion efficiency can be obtained on thin film microcrystalline solar cells by adding more titanium dioxide to the white paint. However, thick layers of the order 80 μm or more of paint are required on thin film solar cells which in themselves are only a few micrometers thick.
A very recent analysis of the enhancement of infrared absorption in solar cells and photo detectors has been disclosed by L. Forbes and M. Y. Louie, “Backside Nanoscale Texturing to Improve IR Response of Silicon Photodetectors and Solar Cells,” Nanotech, vol. 2, pp. 9-12, June 2010. Regularly textured surfaces have been described for solar cells, by A. Arndt, J. F. Allison, J. G. Haynos, and A. Meulenberg, Jr., “Optical properties of the COMSAT non-reflective cell,” 11th IEEE Photovoltaic Spec. Conf., p. 40, 1975. The majority of solar cells use single front side textured surfaces. Front side texturing serves to reduce the reflectivity of the silicon surface due to multiple attempts at transmission through the front surface as has been described by A. Arndt. U.S. Pat. No. 5,589,704 to Levine, “Article Comprising a Si-based Photodetector,” describes the front side texturing of a photodetector by plasma etching. Surface texturing of the illuminated side of solar cells, in the form of a regular, saw-tooth pattern has also been described in U.S. Pat. No. 5,641,362 to Meier, “Structure and Fabrication Process for an Aluminum Alloy Junction Self-aligned Back Contact Silicon Solar Cell”. Further, U.S. Pat. No. 7,582,515, to Choi et al. “Multi-Junction Solar Cells and Methods and Apparatus for Forming Same” discloses a tandem configuration, which is a single piece of semiconducting material. A tandem arrangement as described herein means a single substrate of semiconductor material in which processing has created various layers having individual characteristics of optical, electrical, and/or mechanical properties. U.S. Pat. No. 5,627,081 to Tsuo, et al., describes porous silicon structures on the front side of substrates to reduce reflectance of visible light. U.S. Pat. No. 4,673,770 to Mandelkorn “Glass sealed silicon membrane solar cell” describes a ground and silvered bottom glass cover plate separated from the substrate but not structures etched into or deposited on to the substrate. U.S. Pat. No. 5,080,725 to Green, et al., “Optical properties of solar cells using tilted geometrical features,” describes ridges and pyramids etched into the substrate producing reflections only at specific angles. Matsuyama et al., in U.S. Pat. No. 6,072,117, to Matsuyama et al. “Photovoltaic Device Provided with an Opaque Substrate Having a Specific Irregular Surface Structure”, disclose solar cells CVD deposited upon opaque substrates with linear recesses.
From the foregoing it is apparent that there are conflicting requirements for the absorption of infrared light energy and visible light energy. Front side texturing is desirable to maximize absorption of visible light energy and the backside condition is irrelevant since visible radiation is strongly absorbed and utilized near the front surface. On the other hand, a smooth front side and textured backside is desirable to maximize the utilization of infrared light energy.
Front side textured anti-reflecting layers, or anti-reflecting layers on top of front side texturing are required to maximize absorption of visible light energy, the backside condition is irrelevant since visible radiation is strongly absorbed near the front surface. On the other hand a smooth front side and textured backside is needed to maximize the utilization of infrared light energy. Front side anti-reflecting layers that transmit visible radiation can be used in conjunction with backside diffusive texturing since these front side layers will be reflecting in the infrared.